You can do it in any campaign, but the ones with open areas are better.
Pick up and save the Bile Vial until the point where you get grenade launcher. Wait for new zombie wave then throw the Vile preferably in dead end area or corner. If you play alone you should throw it farther from you since the bots will kill all the zombies on sight.
Wait for some time and when you feel there is enough zombies in one place shoot the grenade launcher. You'll get the achievement. Easy!
It may require couple of tries but this way is still better than throwing pipe bomb since bile vial gives you twice as much time than the pipe bomb to gather zombies around. Plus it won't kill all those precious zombies you need to kill all by yoursefl! :)